1. Field of the Invention
This invention relates to a process for reducing the emissions of sulfur oxides from the regenerator of a catalytic cracking unit. More particularly, the invention relates to a selective removal of a portion of the sulfur from sulfur-containing coke deposits on deactivated cracking catalyst by reaction of these deposits with limited amounts of molecular oxygen in a stripping zone.
2. Description of the Prior Art
A major industrial problem involves the development of efficient methods for reducing the concentration of air pollutants, such as sulfur oxides, in waste gas streams which result from the processing and combustion of carbonaceous fuels which contain sulfur. The discharge of these waste gas streams into the atmosphere is environmentally undesirable at the sulfur oxide concentrations which are frequently encountered in conventional operations. The regeneration of cracking catalyst which has been deactivated by coke deposits in the catalytic cracking of sulfur-containing hydrocarbon feedstocks is a typical example of a process which can result in a waste gas stream containing relatively high levels of sulfur oxides.
Catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to useful products such as the fuels utilized by internal combustion engines. In fluidized catalytic cracking processes, high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely-divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated transfer line reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons of the kind typically present in motor gasoline and distillate fuels.
In the catalytic cracking of hydrocarbons, some nonvolatile carbonaceous material or coke is deposited on the catalyst particles. Coke comprises highly condensed aromatic hydrocarbons and generally contains from about 4 to about 10 percent hydrogen. When the hydrocarbon feedstock contains organic sulfur compounds, the coke also contains sulfur. As coke accumulates on the cracking catalyst, the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline blending stocks diminishes.
Catalyst which has become substantially deactivated through the deposit of coke is continuously withdrawn from the reaction zone. The catalyst particles are then reactivated to essentially their original capabilities by burning the coke deposits from the catalyst surfaces with an oxygen-containing gas such as air in a regeneration zone. Regenerated catalyst is continuously returned to the reaction zone to repeat the cycle.
When sulfur-containing feedstocks, such as petroleum hydrocarbons containing organic sulfur compounds, are utilized in a catalytic cracking process, the coke deposited on the catalyst contains sulfur. During regeneration of the coked deactivated catalyst, the coke is burned from the catalyst surfaces which results in the conversion of the sulfur to sulfur dioxide together with small amounts of sulfur trioxide. This burning can be represented, in a simplified manner, as the oxidation of sulfur according to the following equations: EQU S(in coke)+O.sub.2 .fwdarw.SO.sub.2 ( 1) EQU 2SO.sub.2 +O.sub.2 .fwdarw.2SO.sub.3 ( 2)
One approach to the removal of sulfur oxides from the waste gas produced during the regeneration of deactivated cracking catalyst involves scrubbing the gas downstream of the regenerator vessel with an inexpensive alkaline material, such as lime or limestone, which reacts chemically with the sulfur oxides to give a nonvolatile product which is discarded. Unfortunately, this approach requires a large and continual supply of alkaline scrubbing material, and the resulting reaction products can create a solid waste disposal problem of substantial magnitude. In addition, this approach requires complex and expensive auxiliary equipment.
A second approach to the control of sulfur oxide emissions involves the use of sulfur oxide absorbents which can be regenerated either thermally or chemically. An example of this approach to the removal of sulfur oxides from the regeneration zone effluent gas stream in a cyclic, fluidized, catalytic cracking process is set forth in U.S. Pat. No. 3,835,031 to Bertolacini et al. This patent discloses the use of a zeolite-type cracking catalyst which is modified by impregnation with one or more metal compounds of Group IIA of the Periodic Table, followed by calcination, to provide from about 0.25 to about 5.0 weight percent of Group IIA metal or metals as an oxide or oxides. The metal oxide or oxides react with sulfur oxides in the regeneration zone to form nonvolatile inorganic sulfur compounds. These nonvolatile inorganic sulfur compounds are then converted to the metal oxide or oxides and hydrogen sulfide upon exposure to hydrocarbons and steam in the reaction and steam stripping zones of the process unit. The resulting hydrogen sulfide is disposed of in equipment which is conventionally associated with a fluidized catalytic cracking process unit. Belgian Patent No. 849,637 is also directed to a process wherein a Group IIA metal or metals are circulated through a cyclic fluidized catalytic cracking process with the cracking catalyst in order to reduce the sulfur oxide emissions resulting from regeneration of deactivated catalyst.
U.S. Pat. No. 4,153,534 to Vasalos discloses a process similar to that set forth in U.S. Pat. No. 3,835,031, which involves the removal of sulfur oxides from the regeneration zone flue gas of a cyclic, fluidized, catalytic cracking unit through the use of a zeolite-type cracking catalyst in combination with a regenerable sulfur oxide absorbent which absorbs sulfur oxides in the regeneration zone and releases the absorbed sulfur oxides as a sulfur-containing gas in the reaction and steam stripping zones of the process unit. The sulfur oxide absorbent comprises at least one free or combined element selected from the group consisting of sodium, scandium, titanium, chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc, cadmium, the rare earth metals and lead.
U.S. patent application Ser. No. 91,469 by McHenry (now U.S. Pat. No. 4,276,150) discloses a third approach to the reduction of sulfur oxide emissions from the regeneration of deactivated cracking catalyst. This application is directed to a process for the fluidized catalytic cracking of a sulfur-containing heavy feedstock which contains at least a substantial fraction which cannot be vaporized at atmospheric pressure without extensive decomposition such as residuum and whole crude. These low quality feedstocks result in the formation of large quantities of sulfur-containing coke during catalytic cracking which, ordinarily, are substantially in excess of the amount of coke which must be burned in a conventional regeneration zone to provide process heat. McHenry discloses that the coke which is in excess of that required for process heat balance requirements can be removed and converted to a valuable product by gasification prior to subjecting the catalyst to conventional regeneration. The sulfur-containing coke deposits are gasified with oxygen and steam at a temperature of from about 593.degree. to about 1204.degree. C. in a stripper-gasifier to produce a low BTU gas stream comprising hydrogen sulfide, methane, carbon monoxide, hydrogen and carbon dioxide. The resulting low BTU gas is processed separately from the catalytic cracking products and can be passed to an amine absorption unit of conventional design for removal of hydrogen sulfide and traces of sulfur dioxide.
The McHenry application, however, fails to either teach or suggest that the sulfur content of the coke deposits on deactivated cracking catalyst can be seleectively removed by reaction with small amounts of molecular oxygen. The McHenry application also fails to suggest the possibility or desirability of passing a stream of hot regenerated cracking catalyst and/or hot effluent gas from the regeneration zone to the stripper-gasifier. Further, the McHenry application fails to suggest the desirability of gasifying any portion of the coke deposits except when coke production is in excess of that required for heat balance requirements in the cracking process.
Canadian Patent No. 875,528 discloses a method for regenerating a cracking catalyst which involves reacting the coke deposits on deactivated catalyst with a regeneration gas which consists of oxygen and at least one member selected from the group consisting of steam and carbon dioxide at a temperature in the range of about 556.degree. to 816.degree. C. to form an effluent containing carbon monoxide. This effluent is then passed to a reaction zone wherein the carbon monoxide is combined with steam in a water gas shift reaction to produce hydrogen and carbon dioxide. It is further disclosed that the resulting product gases can be treated by conventional techniques to remove carbon dioxide and hydrogen sulfide. The Canadian Patent, however, requires a complete gasification of the coke deposits and also fails to teach or suggest that the sulfur content of the coke deposits on deactivated catalyst can be selectively removed by reaction with small amounts of molecular oxygen.
U.S. Pat. No. 2,398,739 to Greensfelder et al. discloses a multi-staged fluidized process for the regeneration of deactivated cracking catalyst with an oxygen-containing gas. This patent teaches the partial regeneration of spent cracking catalyst in a low temperature regenerator at a temperature between about 538.degree. and 593.degree. C. Partially regenerated catalyst is then passed to a high temperature regenerator wherein regeneration is completed at a temperature of about 677.degree. C. Similarly, published U.K. patent application No. 2,001,545 discloses a two stage regeneration process wherein there is no major evolution of heat from either regeneration stage. These two references, however, contain no teaching or suggestion that the sulfur content of the coke deposits on deactivated cracking catalyst can be selectively removed by reaction with small amount of oxygen. Indeed, these references contain no mention of sulfur or sulfur oxides in any context. In addition, they fail to suggest either the possibility of desirability of passing hot regenerated cracking catalyst and/or hot regeneration zone effluent gas to a stripping zone wherein sulfur is selectively removed from sulfur-containing coke deposits on deactivated catalyst.